Couple things...
-Would definitely recommend using a decent size Goldenrod if you are putting it in the garage.
-I would also use a desiccant device in the safe like you mentioned, but for the same price, you can get a much larger version at Amazon
Eva Dry 500 The Stack on is just a rebrand of this. Even this larger size will likely need to be recharged every 2 weeks. Also, you may may want to get a digital hydrometer to know the relative humidity in your safe. Available for <$15 on amazon/ebay.
-Since you are placing it in the garage that gets moisture on the floor, DON'T place the safe directly on the concrete. Place the safe on something like this
DRIcore available at Home Depot to keep it directly off the moisture.
-Be sure to bolt that safe down. They are much easier to more than people realize.
-I would seriously recommend you disguise your safe inside a cabinet, or even behind boxes to keep people from seeing it from the street.